This invention relates in general to a vehicular braking system, and in particular a vehicle stability control (VSC) system for use in an anti-lock brake (ABS) and traction control (TC) system.
Vehicles are commonly slowed and stopped with hydraulic braking systems. These systems vary in complexity but a base brake system typically includes a tandem master cylinder, pressure fluid conduit arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The master cylinder generates hydraulic forces in both brake circuits by pressing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the pressure fluid conduit in both circuits to actuate wheel brakes and slow the vehicle.
Base braking systems typically use a brake booster to provide a force to the master cylinder which assists the pedal force created by the driver. The booster can be vacuum or hydraulically operated. A hydraulic booster uses pressurized fluid in a brake booster to move the master cylinder piston thereby increasing the master cylinder pressures generated when the driver applies the brakes. Hydraulic boosters are commonly located adjacent the master cylinder piston and use a boost valve to control the pressurized fluid applied to the booster. Typically the boost valve is connected with the booster in the master cylinder assembly and mechanically coupled to the brake pedal for proper operation.
Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive braking pressure thus causing one or more wheels to lock, resulting in excessive slippage between the wheel and road surface. Such wheel lock-up conditions can lead to greater stopping distances and possible loss of directional control.
Advances in braking technology have led to the introduction of ABS systems. An ABS system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range while achieving maximum braking forces. While such systems are typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels.
Electronically controlled ABS valves, comprising isolation valves and dump valves, are located between the master cylinder and the wheel brakes and perform the pressure regulation. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The isolation valves allow brake pressure into the wheel brakes to increase pressure during the apply mode, and the dump valves release pressure from the wheel brakes during the dump mode. Wheel brake pressure is held constant during the hold mode.
A further development in braking technology has led to the introduction of traction control (TC) systems. Additional valves have been added to existing ABS systems to provide a braking system which controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this condition and automatically applies braking pressure to the wheel brakes of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, braking pressures greater than the master cylinder pressure must quickly be available when the vehicle is accelerating.
During vehicle motion such as cornering, dynamic forces are generated which can reduce vehicle stability. A Vehicle Stability Control (VSC) braking system improves the stability of the vehicle by counteracting these forces through selective brake actuation. These forces and other vehicle parameters are detected by sensors which signal an electronic control unit. The electronic control unit automatically operates pressure control devices to regulate the amount of hydraulic pressure applied to specific individual wheel brakes. In order to achieve optimal vehicle stability, braking pressures greater than the master cylinder pressure must quickly be available at all times.
It would be desirable to provide an integrated ABS/TC/VSC braking system which would provide fluid pressures in excess of master cylinder pressure at any time for full ABS/TC/VSC control.